Phosphorus is one of the raw materials which, at present, looks likely to be subject to severe scarcity in the foreseeable future. This has been recognized by the German Federal Government, which therefore stipulated the following in the coalition agreement: “The protection of water bodies from nutrient inputs and pollutants is to be enhanced and put in a legal framework such that undesirable trends are corrected. We will end the deployment of sewage sludge for fertilization purposes and recover phosphorus and other nutrients.” (CDU, 2013). The fact that phosphorus recovery is indeed justified is shown by the fact that estimated world reserves in 1979 were still 50 billion tonnes, which at the rate of consumption at the time corresponded to an availability of 500 years. According to 2012 estimates, the lifetime of exploitable deposits is about 100 years (Ehbrecht, Fuderer, Schönauer, & Schuhmann, 2012).
The recovery of phosphorus in the communal and industrial wastewater and sludge treatment sector can proceed either from the aqueous phase, from the sludge or from the ashes after sewage sludge incineration. The method claimed here and the apparatus claimed for performance of the method have the aim of recovering phosphorus from the aqueous phase and from the sludge in the form of magnesium ammonium phosphate (MAP, struvite).
Apparatuses and methods for the extraction of phosphorus from wastewater are known in the prior art.
In some cases, MAP crystallization is promoted using sand or other minerals (CRYSTALACTOR® process). In the P-RoC process (DE102011016826A1), phosphate-containing mineral phases are crystallized on the surface of porous calcium silicate hydrate (CSH) substrates as crystallization substrate. The phosphorus present in the substrate is recovered in the form of calcium phosphate.
EP1496019 discloses a method and an apparatus for the recovery of phosphate in the form of MAP crystals from wastewater, in which the wastewater contains high concentrations of organic matter, phosphorus and nitrogen. This treatment process includes the biological treatment of a sludge/water mixture, wherein MAP crystals are formed with simultaneous dosage of a magnesium source and they are induced to grow. After the MAP crystals have been separated from the reactor circulation, at least a portion of the sludge is returned back to the reactor. As well as the phosphorus recovery, this method also simultaneously reduces the organic burden.
WO2014/003554A1 teaches an anaerobic reactor for production and collection of struvite with inlets arranged at the top of the reactor and an outlet at the base, wherein the inlets are configured such that a rotary movement of the reactor contents is generated as the reactor is charged, and the struvite formed is moved toward the outlet by means of scrapers at the base.
EP 1786733B1 also teaches a method which, as well as the recovery of phosphorus, has the aim of degrading the organic burden. In this method, the process proceeds under aerobic conditions, with minimization of the growth of nitrifying bacteria through the choice of a suitable hydraulic residence time, in order thus to have the ammonium co-reactant required for the crystallization of magnesium ammonium phosphate available in a sufficient concentration or not to utilize it in any other way.
WO2008/115758A1 discloses a method for the removal of phosphorus and ammonium from an aqueous stream. The phosphorus-containing aqueous stream is contacted with alkali and magnesium in two to five stages connected in series, and the precipitation of struvite is thus initiated. The struvite is drawn off at the base of each stage and introduced into the preceding stage. The phosphorus-containing aqueous stream is conducted from the first to the last stage in countercurrent. The individual stages feature a rising pH compared to the preceding stage, the pH being increased by the metered addition of alkali.
DE102007035910B4 discloses a method and an apparatus for recovery of magnesium ammonium phosphate in sewage sludge treatment. The apparatus consists of a reaction vessel into which digested sludge is introduced, and the latter is mixed with air. Addition of magnesium chloride initiates the precipitation of magnesium ammonium phosphate. The introduction of air firstly strips out the CO2 dissolved in the digested sludge and thus raises the pH. It is known that the precipitation reaction proceeds more readily at higher pH values. At the same time, the introduction of air creates characteristic flow conditions which enable collection of the heavy MAP particles in a calming zone, from which they can be removed from the system.
DE102011112780A1 discloses a method of treatment of sewage sludge having the treatment steps of hydrolysis of the sewage sludge and the digestion of the hydrolysed sewage sludge that has been subjected to the hydrolysis for anaerobic treatment of the sewage sludge, and having a step of separation of phosphate from the at least partly treated sewage sludge. The separation of phosphate follows the treatment step of hydrolysis and precedes the treatment step of digestion of the hydrolysed sewage sludge, and the sewage sludge having a phosphate content reduced by the separation of phosphate is sent to the anaerobic treatment by digestion.
WO 2005/077834 A describes a fluidized bed reactor for wastewater, comprising a reaction tank having three or more regions arranged one on top of another, the cross-sectional area of which increases from the bottom upward, a recirculation loop and a control system for the regulation of the chemical saturation conditions in the lower region of the reaction tank.
WO 2012/119260 A1 relates to a system for the treatment of wastewater having a reactor tank having three or more regions arranged one on top of another, wherein the cross-sectional area of the regions increases from the bottom upward, and wastewater is supplied in a lower region and is recirculated in the reactor tank.
JP H11-290863 A discloses an apparatus for the separation of phosphorus from wastewater having a reactor tank for the precipitation of MAP crystals (magnesium ammonium phosphate crystals), wherein an image sensor, especially a CCD camera, disposed in the reactor tank is used to determine the size and suspension of MAP crystals.
The known apparatuses and methods have one or more of the following disadvantages:                considerable flushing-out of phosphorus-containing fine crystals;        low degree of phosphorus extraction, i.e. only a small proportion of less than 50% of the phosphorus present in the wastewater is extracted;        only wastewater having a low dry matter content DM<3%, such as sludge centrate in particular, is treated;        use of maintenance-intensive flow-oriented plant components;        restrictions in the choice of operating parameters;        elevated energy consumption.        